1. Field of the Invention
The present invention relates to a fastening element having a core zone formed of a relatively hard carbon steel and a skin zone located outwardly of the core zone and formed of a low-carbon austenitic steel alloyed with a first alloy metal.
2. Description of the Prior Art
The fastening element of the type described above such as, e.g., nails, bolts, screws, anchors, and the like are formed of steel, in particular of a high-strength steel, and are used in an attachment technology for securing objects to hard receiving materials such as concrete, metal, stone or for connecting the objects with each other. The fastening elements have a shaft with, if needed, a pointed end and, if needed, a head provided at the opposite end and having an increased, in comparison with the shaft, diameter. The driving of the fastening element or the attachment process is carried out, e.g., by applying blows to the fastening element, e.g., with a combustion operated power tool, or by rotating the fastening element, e.g., with a screwdriving power tool, or in any other suitable manner.
The conventionally used high-strength steels combine high tensile strength with high yield points (at about more than 800 MPa). As a result, fastening elements, which have a reduced cross-section and, at the same time, a high strength, can be produced. A high-strength steel, however, does not tolerate a corrosive action well. The basis for this is a high predisposition of the high-strength steels to brittle fractures which are caused by stress corrosion when steel is subjected to chemical or electrochemical processes in its regions adjacent to the outer surface. Here, the atomized hydrogen forms an essential component. Hydrogen embrittlement (hydrogen-induced crack formation) takes place primarily in high-strength hardened steels with a tensile strength of about more than 800 MPa. Generally, one distinguishes between a primary hydrogen embrittlement that can take place at a galvanic zinc plating and so-called secondary embrittlement which is also referred to as a corrosion-induced hydrogen embrittlement. Critical parameters here are latent stresses (tensile stresses) which are a result of the high hardness and, e.g., of local inhomogeneity, in materials with a particular structure (martensitic, bainitic, etc.) outside tensile stresses, environmental conditions, and the time factor.
The primary hydrogen embrittlement is treated, as a rube, by heat treatment, a so-called debrittlement. In conclusion of a galvanic refining process, the parts are held at a temperature of about 200° C. for many hours. Part of the hydrogen, which is contained in the steel material, is released, so that the content of hydrogen is decreased below the critical threshold for the fastening element or is distributed to a large extent.
The secondary (corrosion-induced) hydrogen embrittlement occurs in high-strength fastening elements, as a rule, when a fastening element has already undergone a corrosion attack. A typical example of it is the stress corrosion of, e.g., high-strength, galvanically zinc-plated screws and nails used outside in open air. Because this type of the stress corrosion can occur only after a certain “incubation period,” this phenomenon in, e.g., screws and nails became also known as a delayed fracture failure. Therefore, such fastening elements should be used in dry interior spaces and are not suitable for outside use. German Patent DE 38 04 824 C2 discloses a workpiece, which is formed, e.g., as a drilling screw and has a large hardness. The workpiece has a hard core zone formed of a martensitic chrome steel, and an austenitic skin zone formed of chrome, nickel and iron in different combination and having a smaller hardness. In order to obtain this skin zone, the workpiece, which is formed of the chrome steel, is provided with a coating of an alloy containing essentially nickel and cobalt. By a subsequent heat treatment at least 800° C. in an oxygen-free atmosphere and a subsequent diffusion, and austenitic skin zone is produced, and the available hydrogen is expelled. In the produced workpiece, the hydrogen embrittlement should be prevented, and a high corrosion resistance is achieved.
The drawback of these workpieces consists in that the stainless chrome steel is relatively expensive.
An object of the present invention is to eliminate the drawback of the known workpiece and to produce a fastening element having a high resistance against the hydrogen embrittlement and that can be cost-effectively produced.